This invention relates to vehicle intrusion alarm circuits for sounding an audible alarm in response to unauthorized entry of the vehicle. Known prior art intrusion alarm circuits are disclosed in U.S. Pat. Nos. 3,706,966; 3,815,088; and 3,831,141. All of these prior art circuits utilized silicon controlled rectifiers or triacs as the triggering elements for energizing the alarm. However, silicon controlled rectifiers and triacs are quite sensitive to temperature changes and are subject to malfunction at normal environmental temperature extremes. Also, silicon controlled rectifiers and triacs are subject to false triggering due to lightning, electronic ignition on passing vehicles, radiation from nearby radio transmitters, and other outside electrical noises or signals.
The principal object of this invention is to provide an improved vehicle intrusion alarm circuit which is not subject to malfunction at temperature extremes and which is not subject to false triggering by outside electrical noise or signal sources. Other objects and advantages of the invention will be apparent from the description which follows.